When extracting hydrocarbons (oil and gas), the extracted fluid is a mixture of liquid and gas. Typically, the extracted fluid has perhaps 30% liquid and 70% gas (of the volume). However, these proportions are not constant. Furthermore, there may be intermittent bursts or pulses which are almost 100% liquid (known as slugs). These variations in flow cause difficulties in transporting the extracted fluid (e.g. from the extraction point which may be offshore to a delivery point which may be onshore and may be a long way away).
Traditionally separation devices in the form of containers have been used to provide the separation of the multiphase flow. If there is a danger of liquid slugs the container volume must be sufficiently large in order to handle a liquid slug. Such containers are often large and heavy and must be constructed in accordance to design codes for pressure containers. The present applicant has recognised that it would be preferable to provide an inline solution which is built according to pipe code. Pipe and tank codes are concerned with standards and rules for building objects which withstand pressure and the standards and maintenance requirements are different for the same pressure.
A compact degasser (phase splitter) which is designed to fit within a pipe is proposed in WO01/00296. The apparatus separates fluid flow through a pipeline into a heavier and a lighter fraction and is illustrated in FIG. 1a. The apparatus 1a comprises a casing 2a which is connectable within a pipeline (not shown). There is an inlet 3a which receives the multiphase flow, a liquid outlet 4a and a gas outlet 14a. At the upstream end of the casing, there is an axial spin element 5a which rotates and sets the multiphase flow into rotation. The spin element comprises a core body 8a on which there are mounted a number of axially curved guide blades 9a. Upstream of the spin element, there is a guide body 10a supported centrally in the casing by a supporting means 11a. 
The gas is separated into a central zone 6a with the liquid in an annular outer zone 7a. Gas and possibly entrained liquid is discharged through entrance openings 13a in a discharge element 12a. Downstream of the openings 13a is a reflector element 15a and an anti-spin element 16a to bring the rotating liquid phase flow back to an axially directed flow through outlet 4a. The anti-spin element 16a comprises a core body 17a on which there are mounted a plurality of guide blades 18a. There are transfer cones 19a and 20a within the system and a flow restriction 21a at the liquid outlet.
As set out in WO2004/080566 there is a problem with the compact degasser shown in WO01/00296 in that it exhibits problems with slow response, reduce degree of separation and tendency of flooding with liquid. The solution proposed in WO2004/080566 is to provide a control system and a downstream compact multiphase inlet separator with the aim of avoiding gas passage out from the liquid outlet. This is achieved by holding a sufficient liquid volume in the multiphase inlet separator such that a valve in the liquid outlet must be closed before gas is allowed to flow.
An alternative compact separator which is designed to fit within a pipe is a compact deliquidiser as described in WO2002/056999. The apparatus is designed to separate liquid from a multiphase flow and is shown in FIG. 1b. The apparatus comprises a casing 1b which is connectable within a pipeline (not shown). There is an inlet 2b which receives the multiphase flow, a liquid outlet 24b and a gas outlet 3b. In the inlet, there is an axial spin element 4b having an end part 8b with annular lips 9b. The element rotates and sets the multiphase flow into rotation. The gas is separated into a central zone 5b with the liquid in an annular outer zone 6b. There is a gas outlet element 10b having a cylindrical portion 13a which has a central passage 11b connected to the gas outlet and which defines an annulus 12b for liquid flow. The element 10b also has a divergent portion 14b which fits snugly within the casing to form a barrier 15b for water. Water drains into a container 16b at the downstream end of the casing and is drawn out through outlet 24b. 
The upper portion of the container 16b is connected to a central cavity 18b of the spin element 4b by a gas recirculation line 17b. A valve 20b controls flow on the line 17b. 
The spin element 4b has opening 19b for discharge of the recirculated gas. A partition 21b is provided to prevent liquid getting into the gas recirculation line 17b. The bottom 23b of the container 16b is connected to the liquid outlet 24b with a valve 25b which is connected to a gauge 27b and control unit 26b to control flow. There is an anti-spin element 22b in the gas outlet.
Slugs are a particular problem for the compact deliquidiser of WO 2002/056999. The problem of slug inhibition is addressed in WO2004/073829, which proposes the compact cyclone based deliquidizer of WO 2002/056999 with a downstream multiphase inlet separator. In normal operation, the deliquidizer receives a mixed phase flow which contains little liquid and both the liquid and gas from the deliquidizer can flow to the inlet separator. However, if it is detected that liquid is entrained in the gas flow from the deliquidizer, the liquid drain from the deliquidizer bypasses the inlet separator. In this way, the inlet separator is arranged to collect liquid when the draining capacity of the deliquidizer is exceeded.
In both WO2004/073829 and WO2004/080566, the inlet separator is described as being smaller than downstream arranged equipment but must nevertheless have a volume which is such that the volume provides desired residence time for received liquid. Such inlet separators are typically vessel separators. Accordingly, the systems are not compact inline systems.
Other individual compact separators are known from WO2004/000438 to Statoil ASA, EP1600215 to Flash Technologies NV, WO2006/085759 to Flash Technologies NV and WO2009/099339 to Statoil ASA.
A system comprising multiple inline separators is described in WO2009/108063. WO2008/115074 teaches an arrangement having multiple separators which perform gravitational separation. The system is designed and installed according to pipe codes instead of tank (container codes).
Control systems for controlling one or more valves within a system are described in WO2002/046577, WO2007/102079 and WO2008/032201 to ABB and WO01/67189 to Borealis Technology Oy.
The applicant has recognised the need for an improved inline system.